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Can we have it both ways? On potential trade-offs between mitigation and solar radiation management.


Many in the discourse on climate engineering agree that if deployment of solar radiation management (SRM) technologies is ever permissible, then it must be accompanied by far-reaching mitigation of greenhouse gas (GHG) emissions. This raises the question of if and how both strategies interact. Although raised in many publications, there are surprisingly few detailed investigations of this important issue. The paper aims at contributing to closing this research gap by (i) reconstructing moral hazard claims to clarify their aim, (ii) offering one specific normative justification for far-reaching mitigation and (iii) investigating in greater detail different mechanisms that could potentially cause a trade-off between mitigation and SRM. I conclude that the empirical evidence questioning the trade-off hypothesis is inconclusive. Moreover, theoretical reflections as well as economic model studies point to a trade-off. In our current epistemic situation these findings must be taken seriously. They caution against researching and developing SRM technologies before measures to avoid or minimise a trade-off are implemented.


Solar radiation management, mitigation, moral hazard, climate engineering, trade-off


Climate engineering (CE) technologies are commonly classified into methods to remove carbon dioxide from the atmosphere (carbon dioxide removal, CDR) and methods to alter the radiation budget of the earth by means other than changing atmospheric greenhouse gas concentrations (solar radiation management, SRM). The different technologies subsumed under CE vary greatly in their risk profiles. In particular, large-scale methods to reflect incoming solar radiation are associated with considerable risks. Yet some SRM technologies would rapidly lower global mean temperature. Consequently, there is profound disagreement about what their role in combating climate change should be.

Due to the failure of global mitigation policies so far and the potential effectiveness of large-scale SRM in countering a rise in temperature, an increasing number of commentators (most notably Keith et al., 2010) argue that SRM must be studied immediately to get a better idea of the opportunities and hazards associated with it. (1)

One argument warning against negative side effects of such research is the moral hazard argument, which claims that either the deployment or the prospect of deployment of SRM will undermine mitigation efforts. While it is one of the most oft-cited objections to SRM (cf. Rickels et al., 2011: 24) it is regularly dismissed by those advocating extensive research on CDR and SRM (e.g., Blackstock et al., 2009: vi). Although it is often raised, there are surprisingly few detailed investigations of this supposedly important argument (recent exceptions are Lin, 2013; Reynolds, 2014; and Morrow, 2014). This paper aims at contributing to closing this research gap. I will (i) reconstruct and discuss the argument to clarify its aim, (ii) show why it is highly relevant for the current debate and (iii) investigate in greater detail different mechanisms affecting the relationship between mitigation and SRM. To this end, I will highlight--among other things--the intergenerational component of the problem and specify which agents might actually make decisions that lead to trade-offs.

The arguments in this paper rely on three basic assumptions. First, it is still possible to prevent catastrophic climate change through rigorous mitigation--that is, the reduction of anthropogenic greenhouse gas (GHG) emissions. Even 'dangerous' climate change may be prevented if emissions are reduced immediately and substantially (cf. IPCC, 2014: 23-28). Second, atmospheric GHG concentrations cannot be reduced by means of CDR technologies alone while GHG emissions remain high or continue to rise. (2) Third, moving towards a low-carbon society is a realistic scenario; i.e., one that is technologically and economically as well as politically possible. To put it the other way around: if one is convinced that robust mitigation will not take place anyway, the argument of this paper is largely pointless. This, however, is a far-reaching and problematic assumption that is not even shared by proponents of SRM research (cf. section 3). Failure to seriously mitigate would be deeply unjust--whatever else is done. Moreover, this position overlooks both the steps towards a low carbon-economy that have already been taken and the substantial benefits associated with the transformation.


The key concern of the moral hazard argument is that simply researching climate engineering and the prospect of a technical solution to the climate problem could make people less interested in pursuing (more or less painful) mitigation measures (Rickels et al., 2011: 24; Preston, 2013: 3). I agree with Hale that the moral hazard terminology is unhelpful (2012: 130). The moral hazard claim as stated is that introducing SRM will, either inevitably or probably, lead to less mitigation. Such a phenomenon can be adequately captured by the term 'trade-off', which is the term I shall employ. Based on the above statement, the trade-off premise can be stated as follows:

P(TO): If SRM is researched, it will (likely) reduce mitigation efforts.

The idea behind P(TO) is that if it is believed that SRM allows for counteracting the disastrous effects of climate change at comparatively modest costs, societies might be less willing to prevent these effects from happening in the first place by implementing far-reaching mitigation efforts (cf. Burns, 2011: 51). Due to the risks and uncertainties associated with SRM, important reasons to mitigate may remain. This, however, does not militate against P(TO). As plausible as the reasons against less mitigation might be from the point of view of a prudential actor called humanity, they might not be so plausible to many individual present-day actors and lobbies--or may simply be overlooked. To the extent that SRM creates long-term risks or spatially disparate risks, specific actors may (think that they) have good reasons to mitigate less. That is to say, the real question is not so much whether 'humanity' has reasons to do less mitigation but rather how SRM research will impact today's decision-makers: companies, governments, etc.

Moreover, P(TO) is an empirical premise. On its own, it has no implication for what should be done regarding SRM (cf. Morrow, 2014: 2). In order to reach any normative conclusion, P(TO) must be combined with a normative judgment that offers a reason for why far-reaching mitigation ought to happen.


It is often stated that researching/deploying SRM should be no reason to reduce mitigation efforts (e.g., Barrett et al. 2014: 529). There are different ways to justify this and related claims. In the following, I will offer one specific justification for the position that far-reaching mitigation is morally obligatory. The argument developed here is new and links mitigation with potential SRM deployment. If convincing, P(TO) substantially gains in significance. (3)

If the deployment of an SRM scheme is stopped while atmospheric GHG concentration levels are (very) high, the global mean temperature would rise dramatically. McCusker et al. (2014) calculate that if a scheme is employed for 50 years or longer, temperatures will rise with a rate between 0.6 and 2[degrees]C per decade (depending on climate sensitivity) in the first 20 years after its cessation. These results are consistent with those of other studies (e.g., Aswathy et al., 2014) and show that unprecedented and very dangerous warming rates would follow if SRM is stopped and GHG concentrations are high. This is the well-known termination problem. At the same time, SRM technologies are accompanied by other uncertainties and risks. (4) Against this background, William Burns (2011) and Konrad Ott (2012) have argued that the deployment of largescale SRM technologies might put future generations in a dilemma of either terminating deployment and facing a rapid rise in global temperature or coping with unforeseen catastrophic impacts of the technology. This 'dilemma argument' roughly consists of three key premises:

(P1): An SRM pathway might result in a moral dilemma.

(P2): A pathway that might result in a moral dilemma ought not to be adopted if alternative pathways are available that will not result in such a moral dilemma.

(P3): Non-SRM pathways that will not result in a moral dilemma are still available at present.

(C): Therefore, an SRM pathway ought not to be adopted at present.

Instead of discussing the dilemma argument in detail, I will focus on highlighting the relevance for the trade-off argument. The non-SRM pathway in (P3) refers to a combination of stringent mitigation, adaptation and probably some of the less risky CDR measures (cf. Ott, 2012: 37-40). According to climate scientists, it is still possible to avert catastrophic climate change in this way (IPCC, 2014: 23-28). One might further object that stringent mitigation will not happen anyway given political realities. Pointing out that agents will probably continue not complying with their mitigation duties raises important questions regarding non-ideal theory but does not challenge (P3). The dilemma argument is about which strategy ought to be undertaken from the moral point of view. Expected future lack of will to comply with duties to adopt certain strategies (here: mitigation) does not undermine the justification of these duties. It should also be noted that the idea behind (P3) is not that non-SRM pathways are risk-free but rather that they avoid the type of risk involving catastrophic outcomes as outlined by the termination problem. (5)

However, the dilemma argument can be countered in the following way: if accompanied by sufficient mitigation, SRM will not result in the feared dilemma. How much mitigation is 'sufficient' is hard to say, but it is obvious that emissions must be reduced to a very large extent; otherwise GHG concentrations will continue to increase. The uncertainty surrounding the question of how much mitigation will be enough can be captured by describing the situation as follows:

P(DA): The more mitigation is undertaken, the greater the likelihood that SRM will not result in a termination problem with catastrophic outcomes.

Although more can and should be said in defence of P(DA), I assume that it is acceptable to many commentators. In section 7 I will discuss what P(DA) implies in combination with P(TO).

This section has shown that, whatever the merits of SRM, far-reaching mitigation is required, for otherwise GHG concentrations will not go down (see also Preston, this issue). (6) If this is so, it is justified to investigate the plausibility of P(TO) ('If SRM is researched, it will (likely) reduce mitigation efforts') in greater detail. Thus, the next section (4) discusses empirical evidence against P(TO), section 5 discusses trade-offs in economic model studies and section 6 offers a new argument in defence of P(TO). Note that the following three sections pursue two objectives. First, they aim at identifying and discussing the plausibility of different mechanisms that will (not) cause a trade-off (to the extent that this is possible within one paper). Second, they aim at shedding light on the most plausible formulation of P(TO).


I will now address two different issues that are important for the (im)plausibility of P(TO). Rather than discussing these in detail, my aim is to highlight problems with previous arguments. One issue concerns the so-called portfolio perspective, the other issue concerns empirical evidence suggesting that P(TO) is implausible. I will use the Royal Society's recent study (Shepherd et al., 2009) as an example, for it endorses a portfolio perspective and offers some empirical evidence. According to the portfolio perspective, climate engineering technologies are part of a larger portfolio of measures to counteract climate change. Within this portfolio, a combination of different measures can be selected and adapted to changing conditions so that risks, uncertainties and benefits are balanced (ibid.: 56). The portfolio perspective seems reasonable. As there are many uncertainties concerning climate change, it appears that the optimal strategy would be to have many countermeasures that could be combined with one another according to circumstances (Gardiner, 2011b: 175).

However, the portfolio perspective has some shortcomings that are insufficiently acknowledged. (7) The way in which it has been presented by the Royal Society and others (e.g., Keith et al. 2010) suggests that different options are on the table and an informed and morally justified decision about the right mix of responses can then be made. But if there are dynamic relations between different strategies in the portfolio, as claimed by P(TO), their interplay could render the best mix unavailable. That is, if SRM research does undermine mitigation, the mitigation level that turns out to be desirable might not be achievable or an inferior portfolio may be chosen due to an SRM bias (see Morrow 2014). I think that, in principle, these dynamic relationships can be accounted for in a portfolio perspective, although it would complicate things considerably and has not yet been done.

Moreover, the portfolio perspective supposes the existence of a manager or management that prudently optimises the composition of the portfolio over the course of time. However, such a manager does not exist. Rather, decisions about strategies regarding climate change are made by a wide variety of agents that are highly dispersed in place and time. A global body to coordinate climate policy does not exist and even if created would be very different from a commercial enterprise that can quickly decide how to rearrange its portfolio. (8) Finally, as it has been endorsed so far, a portfolio perspective conceals issues of justice and fairness. For example, deciding on the optimal portfolio-mix is usually done from the perspective of humanity as a whole. This fails to account for the fair distribution of burdens and benefits within humanity.

My first challenge of the Royal Society's portfolio perspective is, however, far less relevant if P(TO) is implausible. The Royal Society states that there is hardly any empirical evidence for a trade-off (Shepherd et al., 2009: 39). Furthermore, focus groups conducted in the course of the study would rather indicate the contrary, namely, that efforts in the field of mitigation are expedited when SRM is discussed as an alternative or addition (ibid.: 43). Similarly, a UK study found that participants stated that geoengineering must not distract from mitigation and that both responses should be closely linked (Ipsos MORI, 2010: 53) and Kahan et al. recently observed that 'contrary to the "moral hazard" effect ... subjects in the geoengineering condition ... displayed more concern over climate change than ones in the control condition' (2015: 15).

However, according to Corner and Pidgeon (2014) the situation is more complicated. The extent to which SRM will increase concern for and willingness to undertake mitigation strongly depends on people's value orientations and socio-economic factors. The authors conclude that 'for people who are sceptical about climate change, wealthier and more self-oriented, the prospect of geoengineering may reduce their own motivation to engage in sustainable behaviour' (ibid.: 13).

I will not discuss here which study is more robust. In general, the observation that (some) people show more concern for climate change and mitigation in the face of SRM/CE probably is a good thing. Contra Reynolds (2014: 5-6), this observation does not add much to clarifying the (im)plausibility of P(TO). P(TO) is about how people will act in the future. Stating that one is more concerned about climate change or thinks that SRM requires meaningful mitigation efforts says little about how one will in fact behave and even less about the likelihood of a trade-off in general.

First, what matters is how people are likely to act given the incentive structures they face, given how their peers act, and similar factors. Although people think that mitigation is important, they may not reduce their emissions due to the high costs associated with this and they may not vote for a 'green' party because other issues are more important to these voters than a party's stance on mitigation policies. Or people may think that their individual contribution does not make a difference, or that they have no duty regarding mitigation because they see this as the responsibility of governments and corporations (Baatz 2014). People often show high concern for climate change and mitigation, but this does not translate into corresponding actions for the reasons just stated (among others). That is to say, the fact that people say that mitigation should be undertaken says nothing about whether they also think that they should do something in this respect, and if so, whether they will actually do it.

Second, it is worth noting that in the experiments, scientists transparently disclosed the many risks associated with both climate change and SRM. Such careful framing should not be taken for granted in the public discourse that will actually inform citizens about risks and benefits of mitigation, SRM and other strategies. Lack of public support for climate policies in the USA is certainly connected to the 'careless' framings of the risks associated with climate change in the public debate. It would therefore be important to know how people think about mitigation and SRM in a polarised and biased discourse.

Third, more important than the decisions of ordinary citizens are those of governments, investors and multinational corporations, for they actually affect incentive structures, which in turn affect the decisions of ordinary citizens. Therefore, it would also be good to know how these key decision-makers will in fact behave if and when SRM becomes a serious option. None of the above studies investigates this.

Given these inconclusive findings and given the limited nature of empirical investigations, it is worthwhile discussing other kinds of evidence for or against P(TO).


Another kind of investigation that addresses potential trade-offs between mitigation and SRM is economic modelling. Two standard assumptions regarding SRM are that it will reduce (some of) the negative effects of climate change and that it will be cheaper than mitigation. If so, 'optimal' mitigation levels will be lower in case SRM is deployed because SRM can reduce damages at lower costs. Most studies confirm this basic economic result, while adding further assumptions and complexity.

Juan Moreno-Cruz and Sjak Smulders (2010), for instance, model a 'second-best economy' which includes different countries rather than one prudent decision-maker and in which free-riding on other countries' mitigation efforts is possible. It is assumed that each country chooses its mitigation level to minimise its own costs but takes the mitigation levels of other countries as given. Levels of SRM implementation are also taken as given (in detail, see ibid.: 22-23). Their result is that the implementation of SRM would further reduce mitigation levels due to free-riding behaviour (26). If SRM is employed, damage costs from climate change are reduced and therefore even less mitigation than in the first-best economy will maximise the benefits of each country (23).

In another study, however, Moreno-Cruz (2010) argues that under quite specific circumstances, the availability of SRM results in GHG reductions exceeding the 'optimal' level (18-22). The economic model used by Moreno-Cruz contains two countries and both countries aim at maximising their own benefit. Mitigation exceeds the level that would be 'optimal' from the perspective of the prudent decision-maker, if the countries are affected asymmetrically by climate change and SRM as follows: country 1 is relatively better off with SRM and worse off with climate change and for country 2 it is the other way around (22). Put in very simple terms, country 1 is negatively affected by climate change and better off with SRM. Country 2 expects no or little negative effects from climate change, but fears very negative effects from SRM. In such a scenario, country 1 'could attain a better outcome in a climate negotiation if it could credibly threaten to engage in geoengineering' (ibid.). This is a threat to country 2 because if country 1 implements SRM country 2 would be worse off. Therefore, country 2 has an incentive to mitigate, for otherwise country 1 would resort to SRM. And, since country 1 is better off with mitigation anyway, both mitigate rigorously.

Moreno-Cruz argues that this is a realistic scenario by referring to studies indicating great asymmetries in impacts of both climate change and SRM (21-22). This, however, is not convincing. The fact that SRM and climate change impacts vary greatly is not sufficient. Instead, climate change winners must at the same time be SRM losers and vice versa (ibid.) and, in addition, all those in a position similar to country 1 must have the economic and military power to uphold such a threat. What would world leaders do if the AOSIS group (Alliance of Small Island States) threatened to employ SRM? Millard-Ball (2012: 1060) argues that the 'credible threat from a desperate island nation with no other hope of avoiding annihilation means that other countries' best response is to collectively reduce emissions to the level where it is no longer optimal for Tuvalu to geoengineer'. It is difficult to imagine this happening, given the current world order. Even if the other states hesitate to intervene militarily, the international community could put huge pressure on small and/or poor states by other means (e.g., sanctions). (9) In this situation, world leaders could do many things, but implementing stringent climate policies on the spot is not likely to be the response chosen. My general point here is that at present there is no such constellation of two sets of agents (countries) that fulfils all of the above criteria.

A further serious problem is that in Moreno-Cruz's model countries can free-ride on other countries, but they cannot free-ride on future generations, for a country is supposed to represent the interest of its citizens in perpetuity. However, country 1 and 2 might actually not mitigate because they (i.e., current citizens of the country) are not losers of climate change. Rather, their successors will lose. This is important, for intergenerational free-riding is the most likely form of free-riding (cf. section 6). Taking countries as the central agents fails to acknowledge the intergenerational component of the problem.

In sum, according to economic model studies mitigation will be partially replaced by SRM if it is assumed that (i) the latter avoids or moderates some climate-induced damages and (ii) is cheaper than mitigation. SRM is an imperfect substitute for mitigation, at minimum leading to a partial substitution effect (Goes et al., 2011; Rickels et al., 2011; Emmerling and Tavoni, 2013). When agents are modelled as different players that seek to maximise their own benefit, mitigation efforts will be even further reduced; scenarios that do not exhibit this further reduction are usually unrealistic (cf. also Manoussi and Xepapadeas, 2014). Note that it is possible to model agents differently. Geoschl et al. (2013) assume that current generations care about future generations. In their model, a partial substitution effect can be observed as well, but the effect is limited. In order to avoid huge damages from future temperature increases, current generations undertake substantial mitigation. Among other factors, the higher the climate sensitivity as well as the damages from future deployment of SRM are believed to be, the more mitigation will be undertaken today (ibid.: 95). (10)

Compared to benefit-maximising agents, Goeschl et al.'s assumption is much closer to what moral theory has to say on how people ought to act. What actions can be justified from the moral point of view, however, is not relevant for the plausibility of P(TO). Rather, it matters to what extent agents will act in a self-interested manner. So far, actions of key agents show almost no concern for future generations as well as today's vulnerable. In the following, I will argue that this might not change if SRM is introduced as a further option.


Stephen Gardiner has provided a thorough analysis of the problem of climate change (2006; 2011a). His key thesis is that the 'peculiar features of the climate change problem pose substantial obstacles to our ability to make the hard choices necessary to address it' (Gardiner, 2006: 398). By 'hard choices' Gardiner largely means reducing GHG emissions now and taking on the associated costs. But since the structure of the problem makes us vulnerable to moral corruption this is unlikely to happen. Gardiner's aim is not to demonstrate that it will be impossible to sufficiently reduce GHG emissions. The challenges, whilst significant, can be overcome. He aims to elaborate why it is so difficult to act appropriately. By creating awareness of this, he hopes to make it easier to avoid moral corruption.

Gardiner develops the claim that climate change brings together three different problems, called the 'global storm', the 'intergenerational storm' and the 'theoretical storm'. Each storm generates mechanisms that increase the likelihood that too little mitigation is undertaken. In addition, the three storms reinforce each other, which makes it all the more challenging to reduce GHG emissions (ibid.: 399, 408). The explanatory power of Gardiner's approach is considerable. Although some countries have been successful in lowering their emissions levels, global emissions are still on the rise despite appeals of leading politicians, scientists and civil society as well as 20 years of climate negotiations. Many of the mechanisms identified in the 'perfect moral storm' seem to play an important role in causing the current mitigation failure.

In the following, I will argue that introducing SRM as another option to respond to climate change will either leave the components of each storm unaddressed or reinforce them, thereby making mitigation even more challenging. My argument in this section can be broadly summarised thus: since present low levels of mitigation can be explained by the features of the perfect moral storm that characterise the climate change problem, and since SRM will reinforce these features, SRM will increase the likelihood that mitigation levels remain low. Assuming that Gardiner's analysis has some appeal, I will focus on defending the second premise, which states that SRM will reinforce the features of the perfect storm. Consequently, the argument below is contingent on the first premise being largely correct.

I will now briefly introduce the global and the intergenerational storm as outlined by Gardiner and discuss how each is affected by the emergence of possible SRM strategies. (11)

The global storm and SRM

The global storm and the intergenerational storm involve a spatial and temporal reading of the following characteristics: a) the dispersion of cause and effects, b) the fragmentation of agency and c) institutional inadequacy. The global storm focuses on the spatial dimension (cf. Gardiner, 2011a: 24).

A) Dispersion of cause and effects: Due to the nature of climate change, the impacts of GHG emissions are dispersed around the world. The effects of an SRM scheme would be dispersed around the world as well. This feature alone will not hamper mitigation efforts but neither does it ease the global storm.

B) Fragmentation of agency: Climate change is caused by a vast number of individuals and institutions and although it is collectively rational to mitigate GHG emissions it is individually rational not to do so. Such a situation is usually described as a prisoners' dilemma or a tragedy of the commons (ibid.: 24-28).

The agents researching SRM or conducting such a scheme would be less fragmented than emitters of GHG. On its own, this fact will not make mitigation more likely. It would make mitigation more likely if the wish to carry out an SRM scheme leads to a global regime that also effectively sanctions noncompliance regarding mitigation. This brings us to the next component.

C) Institutional inadequacy: Tragedies of the commons can be resolved by establishing a scheme of cooperation from which all parties benefit. This scheme must allow for punishing free-riding. Otherwise it will be ineffective. In the current international order that consists of sovereign nation states it is very difficult to establish reliable enforcement mechanisms (ibid.: 29).

One of the often-cited advantages of SRM is the possibility of doing it multi- or even unilaterally (Barrett, 2008). It is referred to as an option of last resort if nothing else works. Proponents point out that we should prepare for this case because it seems unlikely that the world community will agree on any meaningful treaty regulating GHG emissions due to diverging interests in the international realm (Victor, 2008). (12) Given that many view SRM as a back-up if global mitigation governance fails, it is hard to see how the prospect of SRM can overcome institutional inadequacy.

Against this, Millard-Ball (2012) argues that the prospect of SRM can be used as a threat in order to increase compliance with mitigation. If A expects severe disadvantages if SRM is deployed and if A can do nothing to stop B from deployment, and if B will deploy SRM when a certain temperature increase is reached, A has reasons to mitigate (if, and only if, this will be able to sufficiently halt temperature increase). Ignoring the fact that the last condition may not be fulfilled in many cases, it is hard to think of an actual country that is in a position similar to B (certainly not Tuvalu, cf. section 5). Perhaps the EU is a likely candidate: it has shown some interest in mitigation and has considerable economic influence. But it is hardly likely at present that the EU would adopt such an aggressive policy. (13) And it is even less conceivable that such a policy would be successful given the current international order.

In general, the above scenario is not realistic at present, for the countries that are most likely to be able to make and uphold an SRM threat are those very countries that are key to the failure of international climate negotiations. Rather, countries such as the US, Russia, and Saudi-Arabia should be induced to mitigate more. I cannot see a coalition of countries that is able and willing to do this. For these reasons, an SRM threat will not be able to spur international agreement on mitigation as long as the international order does not change considerably. Also note that in Millard-Ball's model the only possible response to an SRM threat is mitigation. In reality, it is much more likely that states react with conventional countermeasures such as diplomatic or trade sanctions or military threats--an option that Millard-Ball mentions as well (1063). In sum, since SRM seems unable to attenuate institutional inadequacy and since this inadequacy played an important role in causing current low mitigation levels, it seems very likely that it will also hamper mitigation efforts in future. (14)

Gardiner further argues that there are exacerbating factors that make it more difficult to achieve the global agreement necessary to overcome the institutional inadequacy (see 2011a: 29-32). Such factors also exist regarding SRM.

First, there is considerable uncertainty regarding the impacts of climate change, which might cause states to think that they are better off with (moderate) climate change or at least relatively better off (ibid.: 30). Given that there is much less knowledge of potential consequences of an SRM scheme this adds further uncertainties. Lin (2013: 697) argues that the psychological phenomenon of overconfidence bias 'may lead people to unduly emphasise the dramatic benefits suggested by stratospheric aerosol proposals and to disregard quantitative assessments of risk and uncertainties associated with the technique'. That is, great uncertainty makes overly optimistic assessments of future developments as well as future technologies more likely. Overestimating the possibilities of SRM may cause agents to lessen mitigation efforts because its need is (mistakenly) discounted.

Second, the disparate effects of SRM reinforce what Gardiner calls 'skewed vulnerabilities': since those most responsible for and capable of reducing emissions are not affected first and most severely by climate change, they have little incentives to mitigate at present (Gardiner, 2011a: 31). Regarding SRM, some regions will profit (much) more than others (e.g., Ricke et al., 2010). If powerful countries believe that negative side effects of an SRM scheme will be mostly borne by others, they have reason to think that they will be better-off with SRM (in absolute terms or just relatively). If negative side effects are mostly borne by others, they will lose interest in mitigation. Powerless countries, on the other hand, may lack the means to prevent the powerful from deploying an SRM scheme and very likely lack the power to force them to mitigate. Note that such ruthless behaviour becomes even more likely in the intergenerational storm (see below).

Third, some proponents argue that an SRM scheme would make it possible to avoid the fundamental changes required for the transition to a low-carbon economy, or stretching the process of transition over a longer period of time (Wigley, 2006). It thus relieves industrialised countries of undertaking 'painful' mitigation measures. This shows that the prospect of SRM provides agents not interested in mitigation--most likely those that consider themselves potential losers of a transition to a low-carbon economy--with additional arguments in order to prevent successful mitigation. The opportunity of 'additional arguments' should not be underestimated. In political debates it is desirable that one can respond to journalists and opponents with arguments that sound plausible, at least on first sight, and it is even better if rebuttals of these arguments are long and complicated. Here, the complexity of the perfect storm plays into the hands of those with simple answers (cf. also Lin, 2013: 706-707). (15) In addition to an increase in 'argumentative power', lobby groups not interested in mitigation will also be strengthened in terms of resources, if they are part of research on and development of SRM technologies.

The intergenerational storm and SRM

The three characteristics introduced above have a temporal dimension as well, which Gardiner calls the intergenerational storm.

A) Dispersion of cause and effects: Given the long retention time of GHG in the atmosphere impacts are substantially back loaded (Gardiner 2011a: 33). This creates several problems, including underestimation of impacts, lack of motivation to act, and difficulties adequately preparing for the impacts (34).

An SRM scheme would have rather immediate effects and potentially provide the possibility of fending off disaster at the last minute, so to speak. Proponents cite that as an advantage and I do think they have a point here. However, although the intended cooling effect of an SRM scheme will be noticeable within a couple of years, some unintended side effects might occur after many decades or later. If so, early success of SRM may give a false sense of security and may further undermine mitigation efforts. If GHG concentrations are still rising while SRM is deployed, the termination problem becomes increasingly severe. The dilemma sketched above is largely an intergenerational problem. If current generations invest in SRM technologies but fail to lower emissions correspondingly, this constitutes a risk transfer to future generations. The second feature of the intergenerational storm, fragmentation of agency, increases the likelihood of such a risk transfer.

B) Fragmentation of agency: While spatially fragmented groups of people may still find means to communicate and cooperate to solve a commons problem, temporally fragmented agents do not have this option. This creates the danger of 'intergenerational buck-passing' (ibid.: 35): each generation has strong incentives to not mitigate because the benefits from GHG emissions mostly accrue to them while the benefits from emissions reductions would mostly accrue to future generations. Since each generation faces the same incentive structure, each generation might further contribute to the problem rather than solving it.

I will now argue that the prospect of SRM contributes to this unfortunate incentive structure. As climate change accelerates, substantial warming within this century becomes more likely. Young adults in developed countries have increasing reason to think that they, too, might be negatively affected by climate change at some later stage in their life. And if they will not, their children will likely be so if emissions are not drastically lowered by then. The fact that they (and/or their children) will be negatively affected by climate change--rather than distant successors or people on other continents--increases incentives to mitigate. Now, with SRM as an option, the spectre of dangerous climate change again diminishes to a certain extent. If one assumes that an SRM scheme will be established, e.g., right before climate change becomes 'dangerous', why should one still try hard to reduce one's emissions? Two plausible answers are, among others, to avoid the possibility of a future dilemma and to stop ocean acidification. Both issues will get much more dramatic in the long run, however. It is therefore likely that strong mitigation efforts will benefit future generations in most instances. In consequence, current generations would be mostly excluded from the benefits of mitigation and future generations have no way to coerce current generations to mitigate. In summary, accelerating climate change increases incentives for present generations to mitigate and these incentives are lowered again if SRM is (believed to be) able to avert immediate and drastic impacts of climate change. This increases the likelihood of low mitigation levels.

C) Institutional inadequacy: Political institutions are usually biased towards current interests. The timeframe of democratic decision-making is comparatively short--the next election, a politician's career. Also, implementing an inclusive, democratically ratified system of global governance that has effective sanctioning options will not resolve this kind of institutional inadequacy: the incentive to benefit current citizens (and electorate) at the expense of posterity remains (Gardiner, 2011a: 38). Since SRM can be deployed uni- or multilaterally with respect to technology and costs, it does not seem likely that it will contribute to overcoming institutional deficits. In addition, no institutional design can remove incentives to free-ride on subsequent generations by avoiding costs associated with mitigation.


This section has been based on Gardiner's 'perfect moral storm', which identifies several different mechanisms that lead to less mitigation than is required from the moral point of view and, moreover, less than would be collectively rational for certain sets of agents. I argued that SRM seems unable to attenuate the global institutional inadequacy. Given that this inadequacy played an important role in causing current low mitigation levels, it will also hamper mitigation efforts in future. I further argued that the prospect of SRM will reinforce several of the mechanisms of the perfect storm: First, the uncertainties concerning the distribution of burdens and benefits that would result from an SRM scheme might contribute to an overly optimistic assumption of the net benefits that such a scheme would bring about. This assumption could generate the view that far less mitigation is required than in the absence of an SRM scheme. Second, SRM adds to 'skewed vulnerabilities' in that those who consider themselves to be net beneficiaries of SRM (rightly or not) might show less interest in mitigation, and this is particularly problematic if these 'expected beneficiaries' are powerful countries. Third, SRM would provide arguments and, probably, power to those with vested interests in opposing mitigation policies. Given the past success of 'anti-mitigation coalitions' this should be a serious cause for concern. Fourth, while the intended cooling effect of an SRM scheme will be noticeable within a couple of years, some of the risks associated with it arise in the long run and thus are largely an intergenerational problem. If so, the benefits of mitigation will to a considerable extent accrue to future and not to present generations. This increases incentives for current generations to exploit their temporal position and is highly important, given that it is very hard to prevent intergenerational free-riding. If this argument is plausible, the prospect of SRM will increase the likelihood of continuing high emissions levels.

In the previous three sections I discussed evidence and reasons for and against the possibility of a trade-off between mitigation and SRM. I conclude that such a trade-off is likely, i.e., that the prospect of SRM will probably reduce mitigation efforts and hence emissions reductions. In the next section I will use these findings to specify the trade-off premise P(TO) introduced in section 2 and to discuss what follows from the reasoning so far.


In section 2, I specified the trade-off claim in the following way:

P(TO): If SRM is researched, it will (likely) reduce mitigation efforts.

The mechanisms discussed above mostly build on psychological and economic considerations. On the one hand, the discussion shows that SRM and mitigation are partial substitutes: SRM reduces some of the risks caused by climate change and insufficient mitigation respectively. On the other hand, resources to reduce climate-induced risks are limited. These two basic assumptions suggest that said mechanisms are stronger the more resources are invested into SRM and the more likely deployment is. Rather than being true for 'research' generally, the plausibility of P(TO) probably varies with the intensity and kind of research that is undertaken. For instance, an international research consortium with a billion-dollar budget will have a greater effect than a few research projects at some universities. This suggest that the 'if.., then ... structure' of P(TO) is too simple. More accurate is something like:

P(TO)*: The greater the push towards SRM, the lower mitigation levels will likely be compared to mitigation levels in the absence of a push for SRM. (16)

Note, the formulation 'the push towards SRM' is used as a placeholder for more complex processes, namely that it matters how much money and time is invested into SRM, how many people are concerned with SRM, how visible it becomes, etc.

Current arguments against P(TO)* based on empirical evidence are highly unconvincing. In contrast, economic and moral-psychological considerations indicate that there will be a trade-off. These preliminary results suggest that P(TO)* is plausible. Note, though, that it is not possible to verify/falsify P(TO)* ex-ante. Even ex-post it might be impossible to do so: whether mitigation did (not) occur may have other reasons than the (non-) existence of an SRM scheme and even in retrospect it will be hard to verify to what extent SRM actually did (not) hamper mitigation. One implication is that the theoretical arguments developed above cannot be side-stepped by making reference to empirical evidence. Another is that this kind of evidence must be taken seriously although it may cause some frustration due to its tentative nature.

In combination with P(DA) (cf. section 3) P(TO)* yields the following conclusion:

P(TO)*: The greater the push towards SRM, the lower mitigation levels will likely be compared to mitigation levels in the absence of a push for SRM.

P(DA): The more mitigation is undertaken, the smaller the likelihood that SRM will result in a termination problem with catastrophic outcomes.

C(TO): The greater the push towards SRM, the greater the likelihood that SRM will result in a termination problem with catastrophic outcomes. (17)

The trade-off argument as reconstructed here warns against a serious danger that might emerge if SRM research is pursued. It also says that the more we move towards SRM deployment the greater this danger becomes. This is a significant finding and provides a pro tanto reason to avoid or minimise a trade-off between mitigation and SRM. It provides a strong pro tanto reason, though, for the risk C(TO) warns against is absent in a far-reaching mitigation scenario that is technologically and economically feasible (cf. section 3). (18) This highlights that more time and resources should be spent to remove the political barriers to achieve it.

Measures proposed so far in order to avoid a trade-off either insufficiently tackle the trade-off mechanisms discussed here (Lin, Morrow) or have farreaching consequences that are rejected by many commentators. This paper aimed at demonstrating that there are serious trade-off risks (contra many voices in the debate) and provided a weighty reason for adopting 'anti-tradeoff measures'. What kind of measures can actually be justified by my findings requires another complex debate in which risks and benefits of potential measures must be assessed. This is to be addressed by future research.


I am thankful for feedback from the participants of the ECPR joint sessions workshop 'Climate Change 2.0? Normative and Political Challenges of Geoengineering the Climate' on 11-16 March 2013 in Mainz, from participants of the workshop 'New Debates in Climate Change Justice, Governance and Democracy' on 10-11 July 2014 at Warwick University and from Bertrand Guillaume, my discussant in Mainz. I am also very thankful to Toby Svoboda for written comments on an earlier version as well as two anonymous reviewers for critical but very helpful remarks. Thanks also to Simon Hailwood for help with shortening the paper. Finally, I am highly indebted to Clare Heyward for support and suggestions regarding several versions of the paper.

This work was supported by the Priority Program 'Climate Engineering: Risk, Challenges, Opportunities?' of the German Research Foundation (DFG).


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(1.) The most prominent SRM technology is the injection of sulphate-aerosols into the upper atmosphere. A possible alternative is manipulation of the albedo of clouds. To date, there is little knowledge about this option but early research suggests that marine cloud brightening, like aerosol injections, is highly effective in lowering global mean temperatures, has a possible termination problem, considerable side effects and modest deployment costs (Aswathy et al., 2014). This paper works according to the assumption that all relevant SRM technologies exhibit these features. See also next footnote.

(2.) Current research suggests that several large-scale CDR measures (afforestation, ocean fertilisation, ocean alkalinisation) are either ineffective in lowering global temperatures or associated with risks similar to those of SRM (e.g., ocean upwelling) (Keller et al. 2014). To the extent that a CDR measure shares key characteristics of SRM measures, the arguments in this paper also apply to it.

(3.) Although the claim defended in this section can be found in many contributions to the debate, it is usually stated without a robust justification.

(4.) SRM will very likely cause a worldwide decrease in precipitation and a change in precipitation patterns (Schmidt et al., 2012; Tilmes et al., 2013), though there are considerable variations among different SRM methods (Niemeier et al. 2013). Furthermore, SRM impacts would not be evenly distributed across the globe but affect regional climates differently (Ricke et al., 2010).

(5.) As MacMartin et al. (2014) have recently argued, there will be no termination problem if SRM is only used to reduce peak temperatures. Used in this way, SRM is associated with fewer risks (but cf. Preston, this issue). However, the dilemma argument simply states that SRM makes this kind of dilemma possible, while it is absent in other trajectories. More importantly, in a business-as-usual mitigation scenario the situation may still be very bad, in that severe climate change impacts are only moderately countered by SRM, which additionally causes unintended side effects. That is, even if SRM is only used to buffer peak temperatures, there are equally strong reasons in favour of far-reaching mitigation.

(6.) Note that this leaves room for claiming that slightly lower mitigation levels might be justified if SRM is used. But also note that this claim is different from Reynold's argument (2014: 7, 12) that if SRM reduces the negative effects of climate change and if SRM is cheaper than mitigation, a reduction in mitigation levels can be rational and beneficial because a net reduction in climate risks could be achieved at a lower cost (cf. also Goeschl et al., 2013: 102). He argues from a welfarist perspective that faces severe challenges when applied to cases such as climate change. As is known from discussions of cost-benefit-analysis, calculating welfare levels on global and intergenerational scales requires addressing a long and thorny list of still unresolved methodological and normative problems (see e.g., Hampicke, 2011; Betz, 2006; Randall, 2002).

(7.) For a more detailed critique of the Royal Society's portfolio perspective see Gardiner (2011b).

(8.) A binding global climate treaty limiting overall emissions and governing other technologies/strategies to counter climate change would increase the possibility of 'rational planning' as implied by the portfolio perspective because it would reduce the problem of many uncoordinated decision making processes. Still, the problem that this rational planning must take place in the face of great uncertainty is not diminished. Thanks to Nils Markusson for helpful discussion of this issue.

(9.) Whether more powerful states or collectives might use SRM as a credible threat to increase mitigation levels is discussed in the next section.

(10.) Thanks to Daniel Heyen for a very helpful discussion of the paper.

(11.) I will not discuss whether SRM exacerbates the theoretical storm. First, I do not think the 'theoretical ineptitude' (Gardiner, 2011a: 41) at the heart of the theoretical storm, plays a major role in explaining the current mitigation failure. Second, I do not think that SRM will significantly increase our theoretical ineptitude.

(12.) The issue of legitimacy of an SRM scheme is sidestepped here, but see Svoboda et al. (2011) and Gardiner (2010: 293-294).

(13.) In 2012, the EU dropped its plan to extend the emissions trading scheme to the aviation sector due to international political pressure (Harvey 2012). This illustrates the EU's willingness to adopt confrontational policies.

(14.) One might further object that even if SRM would do nothing to overcome institutional inadequacy, this is not a problem because the motivation to cut emissions is already so low that reducing it further would not matter. This overlooks that i) the transformation towards a low-carbon economy is already underway along with a change in values (WBGU, 2011: 266-267), ii) lack of mitigation policies can be explained to a substantial extent by actions of lobby groups opposed to decarbonisation strategies (ibid.: 189-191) and iii) motivation is not a static phenomenon and changes over time.

(15.) To illustrate what I have in mind, let me give an example discussed by David Luban (2009). In the public debate on torture in the US, proponents of the permissibility of (some forms of) torture by government officials frequently refer to the so-called ticking bomb scenario to justify their claim. In a nutshell, the argument is that a terrorist has planted a bomb that will explode soon and that the terrorist is captured in time but is not willing to reveal where he put the bomb (184). The argument is fraught with highly implausible normative and empirical assumptions. But it takes some time and expertise to show where the claim goes wrong. Too much time for many media users, it seems.

(16.) I am indebted to an anonymous reviewer for help in formulating P(TO)*.

(17.) Many thanks to Frederike Neuber for detailed discussions of the formalisation of the argument.

(18.) Note that the far-reaching mitigation scenario may or may not contain SRM strategies.
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Date:Feb 1, 2016
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